IIT JEE , AIEEE Forum - Chemistry , Organic Chemistry

madhusudan.chavan

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1.Electron donating substituents (EDG) tend to increase and electron withdrawing substituents (EWG) tend to decrease the basicity of aniline & aryl amines.

2.The base weakening effect of EWG & base strengthening effect of EDG is more marked at para position than at meta position.

3.o-substituted anilines are usually weaker bases than aniline regardless of the nature of the group (EDG or EWG).

Groups like ?OCH₃ , -OH , etc., exert both ?I & +R effect. When such a group is present at p-position, +R effect outweighs ?I effect and thus the overall effect is electron-donating. As a result, p-methoxy aniline is a stronger base than aniline.

However, when ?OCH₃ group is present at meta position it can exert only ?I effect, since resonance effect does not occur at meta position. This decreases electron density on nitrogen atom and hence m-OCH₃ aniline is weaker base than aniline.

Thus the order of basicity is

p-OCH₃ (pkb 8.71) > Aniline (pkb 9.38 ) > o- OCH₃ (pkb 9.51) > m- OCH₃ ( pkb 9.80)

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